The present invention relates to a spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship and an iridium (Ir) alloy firing tip provided on at least one of opposing portions of the electrodes. Furthermore, the present invention relates to a method for manufacturing this spark plug.
Spark plugs are employed in internal combustion engines of automotive vehicles, cogeneration facilities and gas compressors. For example, to extend lifetime and improve performance of the spark plug, a spark discharge electrode member, made of platinum (Pt) or Pt alloy, is disposed on at least one of opposing portions of the center and ground electrodes disposed in an opposed relationship.
U.S. Pat. No. 5,456,624 discloses this type of conventional spark plug which uses a rivet platinum firing tip having a head formed at a front end thereof. The head of the firing tip is fixed to an opposing electrode surface by resistance welding.
The spark plugs in future will be subjected to severe engine specifications, i.e., will be used in thermally severe environments. It is predicted that wearability of the firing tip, if it is made of Pt alloy, will be insufficient in such severe conditions. Regarding the wearability, the melting point of iridium (Ir) alloy is higher than that of the Pt alloy. Thus, the iridium alloy is believed to be a prospective material for the future spark discharge electrode member.
The inventors of this application have conducted durability tests on some samples prepared based on conventional spark plug arrangement employing Ir alloy firing tips, with a conclusion that fixation of the Ir alloy firing tip is insufficient according to the conventional spark plug.
More specifically, according to the above-described conventional spark plug, the firing tip is shallowly welded on the opposing electrode surface in such a manner that only a front end or top of the firing tip head sinks in the opposing electrode. If the firing tip is made of Pt or Pt alloy having a linear expansion coefficient similar to that of the electrode base material (Ni-based alloy or the like), it will be possible to obtain sufficient bonding strength and durability.
However, when the firing tip is made of Ir alloy having a linear expansion coefficient larger than that of the electrode base material, the conventional firing tip arrangement cannot assure sufficient bonding strength and durability. In fact, according to an engine test based on practical environments, the firing tip has fallen out of the electrode. Alternatively, it may be possible to use a laser welding for connecting the Ir alloy firing tip to the electrode. However, the laser welding is expensive compared with the resistance welding.
In view of the above-described problems, the present invention has an object to provide a spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship and an iridium alloy firing tip provided on at least one of opposing portions of the electrodes. More specifically, the present invention provides a low-cost method for surely fixing the iridium alloy firing tip to the electrode.
To accomplish the above and other related objects, the present invention provides a first spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip provided on at least one of opposing portions of said electrodes. The iridium alloy firing tip comprises a stem and a head. The firing tip head is integrally formed from one end of the stem with a diameter of the head larger than a diameter of the stem. A distal end of the firing tip stem is opposed to an opponent electrode. The head is embedded in the opposing portion of the electrode. The head is pointed or tapered in a direction opposite to the stem from a maximum diameter portion in such a manner that a cross-sectional area of the head continuously decreases with increasing distance from the maximum diameter portion. The maximum diameter portion is positioned inside the opposing portion of the electrode, and a base end of the stem extending from the maximum diameter portion is wrapped or surrounded by the opposing portion of the electrode.
According to the first spark plug of the present invention, the front end (i.e., top) of the firing tip head is pointed or tapered from the maximum diameter portion in such a manner that the cross-sectional area of the firing tip head continuously decreases with increasing distance from the maximum diameter portion (hereinafter, referred to as tapered configuration of the firing tip head). Thus, when a pressing force is applied during the low-cost resistance welding, the firing tip head can easily sink in the melted opposing portion of the electrode.
According to the embedding arrangement of the first spark plug, the opposing portion of the electrode surrounds or wraps the maximum diameter portion of the firing tip head as well as the base end of the stem extending from the maximum diameter portion. Thus, it becomes possible to securely fix the Ir alloy firing tip to the opposing portion of the electrode in a hooked condition, thereby effectively preventing the Ir alloy firing tip from being mechanically pulled out of the electrode. Hence, the first spark plug of the present invention makes it possible to prevent the Ir alloy firing tip from falling out of the electrode based on low-cost resistance welding.
According to the first spark plug of the present invention, it is preferable that the opposing portion of the electrode surrounds or wraps the firing tip head by a thickness t1 equal to or larger than 0.3 mm. The thickness t1 satisfying this condition assures a sufficient force for fixing the Ir ally firing tip to the opposing portion of the electrode.
Furthermore, according to the first spark plug of the present invention, it is preferable that a pointed or tapered end of the firing tip head is configured into a spherical surface. Alternatively, it is preferable that the pointed or tapered end of the firing tip head is configured into a flattened surface which satisfies a relationship A less than D/2, where xe2x80x9cAxe2x80x9d represents a planar length of the flattened surface and xe2x80x9cDxe2x80x9d represents a diameter of the maximum diameter portion. If the planar length xe2x80x9cAxe2x80x9d is equal to or larger than D/2, the firing tip head will not smoothly sink in the opposing portion of the electrode during the resistance welding operation.
Furthermore, the present invention provides a manufacturing method for a spark plug employed in an internal combustion engine, the spark plug comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip fixed to at least one of opposing portions of the electrodes by resistance welding, wherein the iridium alloy firing tip comprises a stem and a head, the firing tip head being integrally formed from one end of the stem with a diameter of the head larger than a diameter of the stem. This manufacturing method comprises a welding operation for fixing the iridium firing tip to the opposing portion of the electrode by resistance welding. The welding operation comprises a step of bringing the head of the iridium alloy firing tip into contact with the opposing portion of the electrode, and a step of applying a pressing force to the head during resistance welding operation for enforcing a maximum diameter portion to sink in a melted portion of the electrode until a base end of the stem extending from the maximum diameter portion is embedded in the melted portion of the electrode.
According to the manufacturing method of the present invention, the above-described first spark plug can be manufactured adequately. During resistance welding operation, the firing tip head sinks in the melted portion of the electrode when it thermally deforms due to welding heat. This makes it possible to surround or wrap the base end of the stem extending from the maximum diameter portion. Alternatively, according to the present invention, it is possible to caulk the opposing portion of the electrode after the firing tip head is embedded in the opposing portion of the electrode.
Furthermore, the present invention provides a second spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip provided on at least one of opposing portions of said electrodes. The iridium alloy firing tip comprises a stem and a head. The firing tip head is integrally formed from one end of the stem with a diameter of the head larger than a diameter of the stem. A distal end of the firing tip stem is opposed to an opponent electrode. The firing tip head is placed in a hole formed in the opposing portion of the electrode. The head has a maximum diameter portion positioned in the hole. The opposing portion of the electrode is caulked so that a base end of the stem extending from the maximum diameter portion is wrapped or surrounded by an inside wall of the hole.
According to the second spark plug of this present invention, the firing tip head is securely fixed in the hole with the maximum diameter portion placed in the hole and the stem surrounded by the deformed inside wall of the hole. Accordingly, it becomes possible to securely fix the Ir alloy firing tip to the opposing portion of the electrode in a hooked condition, thereby effectively preventing the Ir alloy firing tip from being mechanically pulled out of the opposing portion of the electrode. Thus, according to the second spark plug of the present invention, it becomes possible to effectively fixing the Ir alloy firing tip to the electrode based on low-cost caulking operation.
Furthermore, the present invention provides a third spark plug for an internal combustion engine comprising a center electrode and a ground electrode disposed in an opposed relationship, and an iridium alloy firing tip provided on an opposing portion of said ground electrode. The iridium alloy firing tip is provided on an opposing portion of the ground electrode. The iridium alloy firing tip comprises a stem and two heads formed at both ends of this stem, with a diameter of each head larger than a diameter of the stem. A through-hole, provided on the opposing portion of the electrode, accommodates the stem. Two heads are fixed to the opposing portion of the electrode so as to close each end of the through-hole.
According to the third spark plug of this present invention, the heads are fixed to the opposing portion of the electrode so as to close the both ends of the through-hole. Thus, it becomes possible to securely fix the Ir alloy firing tip to the opposing portion of the electrode in a hooked condition, thereby effectively preventing the Ir alloy firing tip from being mechanically pulled out of the opposing portion of the electrode. This arrangement can be easily realized, for example, by inserting an Ir alloy tip rod into a through-hole and deforming a protruding portion of the tip rod to form the firing tip head. Thus, according to the third spark plug of the present invention, it becomes possible to effectively fixing the Ir alloy firing tip to the opposing portion of the electrode based on low-cost operation.
According to the present invention, the iridium alloy firing tip chiefly contains iridium with at least one additive selected from the group consisting of rhodium (Rh), platinum (Pt), ruthenium (Ru), palladium (Pd) and tungsten (W).